A known liquid jetting device comprises a plurality of ejection units each of which is arranged to eject a droplet of a liquid and comprises a nozzle, a liquid duct connected to the nozzle, and an electro-mechanical transducer arranged to create an acoustic pressure wave in the liquid in the duct.
The electro-mechanical transducer may for example be a piezoelectric transducer forming a part of the wall of the duct. When a voltage pulse is applied to the transducer, this will cause a mechanical deformation of the transducer. As a consequence, an acoustic pressure wave is created in the liquid ink in the duct, and when the pressure wave propagates to the nozzle, an ink droplet is expelled from the nozzle.
EP 1 378 359 A1 and EP 1 378 360 A1 describe ink jet printers which comprise an electronic circuit for measuring the electric impedance of the piezoelectric transducer. Since the impedance of the transducer is changed when the body of the transducer is deformed or exposed to an external mechanical strain, the impedance can be used as a measure of the forces which the liquid in the duct exerts upon the transducer. Consequently, the impedance measurement can be used for monitoring the pressure fluctuations in the ink that are caused by the acoustic pressure wave that is being generated or has been generated by the transducer.
The impedance measurement may be performed in the intervals between successive voltage pulses. In that case, the impedance fluctuations are indicative of the acoustic pressure wave that is gradually decaying in the duct after a droplet has been expelled. This information may then be used for adapting the amplitude of the next voltage pulse, for example.
As has been described in EP 1 013 453 A2, the impedance measurement and the monitoring of the pressure wave in the duct may also be utilized for detecting a breakdown of the ink duct without interrupting the operation of the printer. For example, air bubbles in the ink duct will cause a characteristic signature in the decay pattern of the acoustic wave. Similarly, if the duct is (partially) clogged by a solid particle, this will result in an impedance signal having a lower frequency, a smaller initial amplitude and a stronger damping characteristic.
In the known devices, the measured impedance and the resulting pressure signal are utilized only for controlling the very transducer from which the pressure signal has been obtained. The parameters that are controlled on the basis of the pressure signal relate only to the amplitude and/or shape of the pulses with which this individual transducer is energized. Other operating parameters, in particular the drop generation frequency which determines the printing speed, have to be the same for the transducers of all injection units.
When printing with a high drop generation frequency, a high image quality can be expected only on condition that there is a suitable match between the configuration of the ejection units and the acoustic properties of the ink. If, for example, the viscosity of the ink is not in a suitable range, this may lead to undesired pressure fluctuations in the ink and to cross-talk among neighbouring ejection units, so that the image quality will be compromised.
It is generally known in the art that the control system of the printer may automatically detect the type of ink being used, e.g. on the basis of certain marks on the ink cartridge, and shut down the printer if the ink is not of the correct type. It may also be conceived that the printer is operated with a lower drop generation frequency if the ink is not of the correct type.